Dry-compression vacuum pumps are pumps which have oil-free pump chambers. The advantage offered by these pumps is that they are capable of generating a vacuum which is free of hydrocarbons. Therefore, they are preferably employed for the evacuation of chambers in which etching, coating or other vacuum treatment or vacuum production processes are run. In particular, Roots pumps or multi-stage claws pumps are well-proven in practice as dry-compression pumps, i.e., dual shaft vacuum pumps having one or several stages. A pair of rotors is located within each stage. The rotating motion with respect to the rotors themselves and with respect to the walls of the pump chamber is such that no contact is established.
When employing dry-compression pumps in connection with the processes mentioned above, or also for evacuation of vacuum chambers in which chemical processes are run, it is often the case that, depending on the type of application, solids or liquids enter the pump. As long as these are only pumped through the pump, they will hardly impair operation of the pump. However, generally these substances are chemically aggressive or at least chemically or physically reactive, so that these may cause abrasions (in the case of liquids, for example) or the formation of layers (in particular, when dust is in involved). In both cases there exists the danger of reduced service life for the vacuum pump. In the presence of aggressive liquids, wear effects are increased. The danger of dust exists in particular when the pumping semiconductor process gases. In this kind of application the circumstances may be such that extremely fine solid particles may be formed during the compression phase of the gases, i.e. when the gases which are to be removed pass through the vacuum pump. Solid particles which enter the vacuum pump either directly or indirectly may deposit themselves in the pump chamber, where they at first only cause a narrowing of the slit between the rotors. Further deposits may cause the rotors to touch, so that the solid particles are rolled on to the surfaces of the rotors. When the deposits increase further, then the layer which is rolled on to the rotors thickens, so that a force is created which forces the rotors and thus the shafts of the rotors apart. In particular, when the rolled on layer increases further, this will damage the bearings and result in a failure of the pump.